A number of species engage in swarming behavior, where countless individuals move en masse in some direction. But they're not the only ones. Scientists have now found that the sperm of some squid form swarms, and this peculiar behavior could help the cells to fertilize eggs.

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Sexual dimorphism — where males and females of a species exhibit different traits — is widespread in the animal kingdom. For example, female widow spiders are much larger than their mates, and only male mallards sport the glossy green heads that the species is known for. It's way less common to see a single sex of a species shows strong dimorphic traits — but that's exactly what we see with the spear squid Loligo bleekeri.

In this species, which is also known as Bleeker's squid, males can either be large "consorts" or small "sneakers." And size isn't the only thing distinguishing the two classes of squid; they also go about mating differently. Consorts compete with other males and court females by flashing vibrant colors across their bodies. A successful male will place his spermatophore (sperm packet) into the female's oviduct, an internal tube leading to her ovaries. The male then earns his consort title, by guarding over his mate until she deposits her eggs.

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A sneaker male won't waste time with complicated courtship rituals. Instead, he will rush headfirst at a mating pair and place his spermatophore near the female's external sperm storage organ, just under her mouth. When the female spawns, her eggs travel through her oviduct to a spot near her mouth. There, the sneaker's sperm can fertilize eggs that weren't already fertilized by consort sperm inside the oviduct (sneaky, right?).

Smaller squid make up for their size by producing bigger sperm than their larger competitors.…
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But this discovery left the scientists wondering what other interesting properties the squid sperm may have. They decided to test whether the sperm form motile clusters — that is, if they move together as a group, rather than diffusing. In polyandrous species (where females mate with multiple males), related sperm will sometimes aggregate, allowing their mass to outswim the individual sperm of rival males.

Only the sperm of the sneaker squid form swarms (seen in this video as glowing bands). Courtesy of Cell Press/Elsevier.

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For their new study, the researchers began by placing L. bleekeri sperm into capillary tubes. The sperm did cluster together, but only if they belonged to sneaker squid (video above). What's more, the swarming sperm weren't binding together — they swam individually. Further experiments showed that the sneaker sperm tried to swarm not only with other sneaker sperm, but also with consort sperm and sperm from the starfish Asterina pectinifera.

"It was very weird, very strange," says lead researcher Noritaka Hirohashi, a marine biologist at Shimane University in Japan. Hirohashi and his team figured that the sneaker sperm must be attracted to some kind of chemical the sperm were releasing. "I started thinking about cellular respiratory products — carbon dioxide is one of them," Hirohashi tells io9.

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The team conducted several experiments to see if CO2 is truly behind the swarming behavior. In one test, they placed a bubble of CO2 into a tube and saw that sneaker sperm (but not consort sperm) swarmed around it (video below). In another test, they injected a compound into seawater that releases CO2 when hit with UV light — the sneaker sperm migrated to whatever region of the tube that was activated with the light.

The sneaker squid sperm are attracted to CO2 bubbles. Courtesy of Cell Press/Elsevier.

Importantly, other tested chemicals didn't elicit the same response from the sneaker sperm. The scientists also found that they could stop the sperms' swarming behavior if they inhibited the cells' ability to sense CO2. Taken together, the results suggest that the sperm sense CO2 and swarm towards the strongest source (each other, if no other source is around).

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So what's behind this behavior? "I don't know yet why sperm sense CO2, in terms of what benefits that provides," Hirohashi says. But the team isn't without ideas.

For one thing, if the sneaker sperm diffused after being deposited, they would have a reduced chance of fertilizing eggs. "When the male deposits his sperm onto the female's body, the sperm has to wait until the female ovulates," Hirohashi explains. Sneakers deposit their sperm before females release their eggs — swarming behavior may help keep more sperm cells in the correct area to fertilize the eggs. This behavior, however, wouldn't be much use to consort sperm.

The sperms' penchant to swarm towards CO2 may also have to do with the eggs. In their research, the team saw that the sperm are attracted to the eggs. This attraction could mean that the eggs are releasing CO2, drawing the sneaker sperm to them.

The team is now looking to test these ideas and figure out exactly what's behind the sneaker sperm behavior. They also want to find out if sneaker sperm only produces sneaker squid and if consort sperm only produces consort squid. Who knew that sperm could be so fascinating?